1. Field of the Invention
The present invention relates to a device for reading magnetic information. It is applicable in particular to apparatus for reading checks.
2. Description of the Prior Art
It is known that present-day data processing systems frequently include data infeed arrangements which are called "readers" which employ slips carrying coded magnetic information. These slips are for example, bank checks, post-office checks, withdrawal slips, deposit slips, credit cards, or the like.
The information concerned generally consists of a series of alpha-numeric characters printed on the slips, that is to say a succession of letters of the alphabet, figures, punctuation marks, etc. which indicate, in the case where the slip is a check for example, the number of the check or the account number of the drawer.
Each character is formed by a set of bars composed of magnetic ink. The number of bars, the distance between the bars, and their relative disposition are individual to each character and are coded in accordance with known codes such as, for example, the CMC 7 code.
The check reader converts the coded magnetic information represented by the characters printed on the check into a succession of electrical signals of constant amplitude and variable length. These signals are transmitted to electronic shaping circuits where they are converted into a series of square-wave electrical pulses of constant width and amplitude. These pulses are in turn transmitted to electronic circuits for recognizing the characters printed on the check. As soon as the characters corresponding to the series of electrical pulses have been identified, it is possible to cause a calculating unit in the data-processing system with which the check reader is associated to perform operations relating to the check such as debiting, crediting and updating the account.
So that the subject of the invention may be better understood, a brief review of some facts relating to magnetism will be given.
To magnetize a magnetic material in which the magnetic induction is low, the material is first subjected to a positive magnetic field whose strength is sufficient to saturate the material, that is to say for the magnetic induction in the material to reach a limiting value Bs. The external magnetic field is then removed. There then remains in the material a magnetic induction (+Mr) termed the residual induction which is other than zero and which is characteristic of the material. The B/H ratio is termed the initial magnetic permeability of the material when H approaches zero.
If a negative magnetic field is applied to this magnetized material, the magnetic induction in the material becomes zero at a value of H which is termed the coercive field Hc.
In the immediate vicinity of its surface, a magnetized magnetic material creates a magnetic leakage field which is a function of the coercive field Hc and which is theoretically equal at most to this coercive field. In practice the leakage field is always less than the coercive field.
A material which is magnetically anisotropic has two preferred directions of magnetization which are perpendicular to one another. One of these is termed the direction of easy magnetization, while the other is termed the direction of hard magnetization. The initial permeability of the material in the direction of hard magnetization is much higher than the permeability of the material in the direction of easy magnetization. What is termed the anisotropy field Hk is that value of the magnetic field H applied to the material at which the material is saturated in its direction of hard magnetization.
The magnetic flux .phi. of a magnetic field H through an area S is equal to the product of the strength of the field and the size of the area.
Check readers generally include a magnetizing device to magnetize the bars forming the printed characters on the check in order to render the value and sense of magnetic induction identical in all the bars. Such magnetization is necessary because when the characters are printed on the check, either the induction in the bars is zero or else the value and sense of the magnetic induction in all the bars varies from one bar to the next. Thus, the magnetic induction in the bars is equal to the residual induction of their magnetic ink when they are no longer subject to the magnetizing field of the magnetizing device. Check readers also include generally a magnetic transducer device which is sensitive to the magnetic leakage field generated by the bars after they have been magnetized by the magnetizing device. The transducer device emits in response to this field, an electrical signal which is transmitted to the aforementioned electronic shaping circuits. Thus, the magnetic transducer device enables the presence of bars to be detected.
The check is moved by a mechanical transporting device and is positioned in the device in such a way that all the bars initially pass in front of the magnetizing device and then the magnetic transducer device, in succession, and in close proximity thereto. The mechanical device is driven either manually or by an electric motor.
Simple and inexpensive magnetizing devices are known which are extremely effective in saturating the bars. Such a device is described, for example, in U.S. application Ser. No. 880,331, filed in the name of Jean-Pierre Lazzari and Michel Helle on Feb. 23, 1978, and entitled "A Localized-Field Static Magnetizing Device." Said application is assigned to the assignee of the subject application and the subject matter thereof is incorporated herein by reference. Such a device consists of a magnetic layer preferably having a high coercive field which is deposited on a preferably insulating substrate, the layer containing a succession of adjoining zones which are of different alternating lengths d1 & d2, one of which is very much greater than the other, and whose magnetic inductions are of opposed senses and are generally equal in value to the residual induction of the magnetic layer. Such magnetizing devices have a high magnetic leakage field which is very much confined to the vicinity of the surface and to the edges of the magnetic layer.
Also known are simple and inexpensive transducer devices which are highly reliable in detecting the presence of bars, while at the same time being insensitive to their speed of movement. Such magnetic transducer devices generally employ magneto-resistors.
It will be recalled that magneto-resistors are electrical resistors which are deposited on a substrate of insulating material in the form of thin layers or films of very small thickness (from a few hundred angstroms to a few microns in thickness) and whose resistance varies when they are subjected to a magnetic field. A measuring magneto-resistor R of this kind is connected to the terminals of a generator which outputs a current I. When a bar passes in front of the magneto-resistor, the flux of the magnetic leakage field H of the bar causes a change .DELTA.R in its resistance and thus a change in voltage .DELTA.V=I .DELTA.R at its terminals, .DELTA.R/R being termed the coefficient of magneto-resistance. U.S. application Ser. No. 899,383, corresponding to French patent application No. 77.13026 entitled "Magnetic Transducer Device For Detecting Coded Magnetic Information And Method Of Manufacturing The Said Device" which was filed in France by the applicant's present assignee on Apr. 29th 1977, discloses a magnetic transducer device which contains at least one magneto-resistive member for detecting the presence of bars and a magneto-resistive compensating member which is arranged close to the magneto-resistive detecting member in such a way that it is not subject to the magnetic leakage field of the bars, but is subject to the same interference magnetic fields as the detecting member. Said U.S. application Ser. No. 899,383, corresponding to French application 77.13026, was filed in the United States in the name of Jean-Pierre Lazzari and Michel Helle on Apr. 29, 1977 and is assigned to the assignee of the subject application, the subject matter of said application is hereby incorporated herein by reference.
What are termed interference magnetic fields are magnetic and electromagnetic fields other than the magnetic leakage field H of the bars, such as the magnetic fields which are generated by any nearby electrical apparatus and the magnetic field of the earth.